The present invention relates to vector network analyzers, and more particularly to an improved calibration method for a vector network analyzer.
To obtain accurate results the users of vector network analyzers calibrate the instruments by measuring three known impedance standards, typically a short, an open and a characteristic impedance (Z0) load for one-port measurements. For two-port measurements three additional measurements are performed using non-connection and through connections between the ports. Using the measured results of these measurements, the vector network analyzer""s systematic errors are mathematically corrected, resulting in excellent measurement accuracy. Measurements consist of many stepped measurements at sequential frequencies that appear to be xe2x80x9csweptxe2x80x9d when a user is measuring a Device Under Test (DUT). Each measurement point has xe2x80x9ccalibrationxe2x80x9d data taken during the calibration routine that are stored and used for correcting that particular measurement point.
The problem is that, while the vector network analyzer gives excellent results, the calibration is done at each exact frequency step used in the measurement. For example if a vector network analyzer has a potential frequency measurement range of 25 MHz to 2500 MHz with 100 kHz frequency steps, then to gather calibration data for each frequency step requires 3xc3x9724,751=74,253 calibration measurements without taking multiple measurements for noise reduction. This large number of measurements requires an inordinate amount of time for calibration procedures. So instead the calibration is done over a specified measurement range, such as 500.5 MHz to 1011.5 MHz, which only requires 3xc3x975, 111=15,333 calibration measurements. However anytime that any frequency parameter, such as start frequency, stop frequency, number of frequency points, frequency resolution, etc., is changed by a user, the vector network analyzer must be re-calibrated. Many users do not need measurements of extreme accuracy, and they find re-calibrating the vector network analyzer each time a frequency variable is changed, even slightly, to be very cumbersome and time consuming. Therefore the user has the choice of either operating without calibration at all or of taking the time and effort of having excellent calibration.
Also prior vector network analyzers require low-phase-noise and low-amplitude-noise measurements, particularly for the calibration measurements. If there is any significant noise, as might be the case with low-cost hardware in the vector network analyzer, then many measurements are taken and a large amount of averaging is used to reduce the effect of the noise, adding another multiplier to the number of calibration measurements that need to be taken.
What is needed is a vector network analyzer that provides accuracy as well as ease of operation, even when low-cost hardware is used.
Accordingly the present invention provides an improved calibration method for a vector network analyzer that acquires sparse calibration data across the frequency range of the vector network analyzer, or at least a larger range than a specified measurement frequency range. The sparse calibration data may be obtained by measuring every Nth frequency step of the vector network analyzer, or by measuring each frequency step of the vector network analyzer and compressing the results. Then for each measurement frequency of the vector network analyzer a correction value is appropriately interpolated from the sparse calibration data to provide calibration error data. The calibration error data is then used to correct the measurement data to provide an accurate result.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.